U.S. intelligence officials warned Wednesday that the government shutdown is making the nation more vulnerable to terrorist attacks and other security risks by forcing U.S. spy agencies to send tens of thousands of employees home.

Director of National Intelligence James R. Clapper Jr. testified that the furloughs of civilian employees could have an “insidious” effect, degrading intelligence-gathering capabilities in ways that may not become fully clear for weeks or months.

“As each day goes by, the impact and the jeopardy to the safety and security of this country will increase,” Clapper told the Senate Judiciary Committee, testifying alongside Gen. Keith B. Alexander, director of the National Security Agency.

Many analysts and operatives in critical assignments on counter­terrorism and other issues are considered “essential” employees and are, therefore, protected from the furloughs, meaning that the CIA and other agencies are continuing to track al-Qaeda-related threats, monitor watch lists and perform other urgent tasks.

But figures released this week by Clapper’s office indicate that 72 percent of the intelligence community’s civilian workforce has been temporarily sent home, creating holes in virtually every agency and department.

At the CIA, the furloughs have affected nearly every job category, officials said, with the impact heaviest on employees in administrative or support functions. Because of its role, the CIA is thought to have been able to keep a larger percentage of its employees than other agencies, but the shutdown has forced analysts and case officers in certain categories to temporarily leave their jobs.

The CIA and other agencies declined to provide details on the furloughs, saying that the information is classified and could identify vulnerabilities that adversaries might seek to exploit. Some agencies, including the NSA, rely heavily on military personnel who are not affected by the furloughs.

Sen. Dianne Feinstein (D-Calif.), who chairs the Senate Intelligence Committee, said in a speech on the Senate floor Tuesday that the furloughs would “cripple” spy agencies and put U.S. forces and diplomats overseas at greater risk.

“Our shutdown is the biggest gift that we could possibly give our enemies,” Feinstein said. With few exceptions, “the lights are being turned off and the majority of the people who produce our intelligence, analyze intelligence and provide warning of terrorist attacks or advise policymakers of major national security events will be prevented from doing their jobs.”

The budget showdown’s broad impact on spy agencies is in contrast to the U.S. military, though the Pentagon also has been forced to furlough thousands of civilian employees.

Sen. Charles E. Grassley (R-Iowa) expressed surprise during Wednesday’s hearing that so many employees of intelligence agencies were considered non­essential under federally imposed criteria that agencies have been forced to interpret.

“I’m concerned that if your lawyers have determined that 70 percent of your employees are nonessential to your mission that you either need better lawyers or to make big changes in your workforce,” Grassley said.

Clapper replied that proving that workers were necessary “to protect against imminent threat to life or property” under the furlough criteria “causes us to make some very, very painful choices.”